Several methods have been employed for forming particulate or powder-like materials into a unitary firmly compacted body of material.
Powdered metal bodies have been formed by means of pressure and heat. Such a method has also been used for forming unitary bodies from other powder or particulate materials.
A problem has specifically existed with regard to forming superconducting powders into a unitary firmly compacted body. Ceramic superconducting powders are normally prepared by proportioning the specific quantities of selected oxides. The combination is then thoroughly mixed by conventional means and then fired at elevated temperatures in suitable gaseous atmospheres. The induced solid state reaction causes the formation of the desired ceramic compositions and lattice structures.
In ceramic superconductors, the superconductivity within individual crystallites is proximity coupled to neighboring grains. Consequently, the orientation and coupling between crystallites are key factors affecting the current carrying capacity of the bulk ceramic superconductors. Voids, cracks, and grain boundaries act as weak links between crystallites and reduce the critical currents within the bulk material. Therefore, a technique which produces dense ceramics with good intergrain coupling and by which the material is formable into desired shapes to yield a required superconducting characteristic is of significant value.
At the present time several methods are used for obtaining high critical current densities in bulk superconducting materials.
One method employed is that of melt textured growth of polycrystalling material. This method is discussed in a paper included in Volume 37, No. 13, May 1, 1988, Physical Review B., S. Gin, et al., entitled: Melt-Textured Growth of Polycrystaline. This method consists of heating a bulk specimen of the high temperature material in a furnace to temperatures at which partial melting occurs. A temperature gradient is maintained in the furnace, and the superconductor is melted and recrystallized as the specimen is passed through the hot zone. Highly textured material is produced through this method and at present is shown to yield high Jc values. This method is generally limited to the processing of small length samples.
Another method is that of placing powder in a tube. This xe2x80x9cpowder in tubexe2x80x9d method is discussed in a paper 1989 Applied Physics Letters, page 2441, prepared by K. Heine, et al., entitled: High-Field Critical Current Densities. In the xe2x80x9cpowder in tubexe2x80x9d method, mechanical deformation is used to align plate-like particles of bismuth based superconductors. The powder is loaded into a tube of silver material and the assembly is compacted by swaging, drawing or rolling. A silver sheath provides a path to shunt currents across any defects. The material is subsequently heat treated to obtain the optimum superconductor characteristics.
However, as a result of the nature of varied mechanical operation involved in the two methods discussed above, reproducing the many processing steps repeatedly during fabrication of long lengths of wires and tapes remains unsatisfactory.
Another method of compaction is that of hot extrusion. This method is discussed in an article entitled: Hot Extrusion of High-temperature Superconducting Oxides by Uthamalingam Balachandran, et al., American Ceramic Bulletin, May 1991, page 813.
Another method is discussed in U.S. Pat. No. 5,004,722, Method of Making Superconductor Wires By Hot Isostatic Pressing After Bending.
Another compaction techniques which has been employed pertains to a shock method. This method is discussed in an article entitled: Crystallographically oriented superconducting Bi2Sr2CaCu2O8 by shock compaction of prealigned powder by C. L. Seaman, et al., in Applied Physics Letters 57, dated Jul. 2, 1990, page 93.
Another method of compaction is that known as an explosive method, discussed in an article entitled: Metal Matrix High-Temperature Superconductor, by L. E. Murr, et al., in Advanced Materials and Processes Inc. Metal Progress, October 1987, page 37.
These methods are limited in value because they are generally applicable only to production of small body sizes.
The application of large uniaxial static pressures at elevated temperatures is discussed in an article entitled: Densification of YBa2Cu2O7-8 by uniaxial pressure sintering, by S. L. Town, et al., in Cryogenics, May 1990, Volume 30.
The use of electromagnetic forming for the purpose of attachment is discussed in a paper entitled: Electromagnetic Forming, by J. Bennett and M. Plum, published in Pulsed Power Lecture Series, Lecture No. 36.
However, processing of long lengths of homogenous and high quality superconducting tapes or wires by the processes discussed above has not been realized.
It is an object of this invention to provide a method and means for producing high density bodies by the use of powder-like and/or particulate materials.
It is another object of this invention to provide a method and means for producing electrical conductors by the use of powder-like or particulate materials.
It is another object of this invention to provide a method and means for producing high quality and continuous superconducting electrical conductors such as wires and tapes.
It is another object of this invention to provide such a method which can be consistently precisely duplicated in the quality of production.
Another object of the invention is to provide a method for magnetically compacting a powder to achieve in excess of 90% of its maximum density using applied pressures which have heretofore not been able to achieve such densities.
Another object of the invention is to provide a method and system for selecting various variables which enable the system and method to compact a material to a density which exceeds densities normally achieved with a given applied input pressure.
Still another object of the invention is to provide a method and system for accelerating a wall of a container for compacting a material to densities in excess of 90% of that material""s maximum density.
Other objects and advantages of this invention reside in the construction of parts, the combinations thereof, and the methods employed, as will become more apparent from the following description.
In this invention, powder-like and/or particulate materials or the like are compacted into high density bodies. The high density bodies can be of various shapes and sizes, and may, for example, be bodies such as rods, tapes, tubes, or plates or any other suitably shaped or desirably shaped bodies.
The method and structure of this invention applies pressures generated by noncontact electromagnetic forces. These electromagnetic pressures are generated by employing suitably shaped coils, such as solenoids or the like which have the necessary current carrying capacity. In this process a suitable electrically conductive container is encompassed by such a coil or solenoid. Within the electrically conductive container powder-like material is enclosed. When high magnitudes of electrical current are passed through the solenoid or coil, very high pressures are applied to the electrically conductive container, and the electrically conductive container is reduced in transverse dimensions. Thus, the powder-like material within the electrically conductive container is compacted into a body of high density.
In one embodiment of this invention superconducting powders are placed upon an electrically conductive strip, and the strip is formed into a tubular member, thus enclosing the superconducting powders. The tubular member is encompassed by a solenoid or coil. High current levels are passed through the solenoid or coil, and a high magnitude of resulting electromagnetic pressure is applied to the tubular member. The transverse dimensions of the tubular member are significantly reduced and the superconductive powder within the tubular member is thus firmly compacted. If desired, this process can be performed in a continuous manner, so that an elongate conductor of superconductive material is produced. The compaction method of this invention is capable of producing wire or tape of normal electrical conducting material or of superconducting electrical materials.